1. Field of the Invention
The present invention relates to a vector controller for an induction motor and, more particularly, to a vector controller for an induction motor that is capable of automatically adjusting a set value of a secondary resistance of the induction motor, namely, a resistance of a rotor of the induction motor.
2. Description of the Related Art
In general, vector control has been extensively used in industrial fields as a method for quick control of an output torque of an induction motor. The following will briefly describe the vector control.
The vector control is carried out to independently control a torque and a secondary magnetic flux of an induction motor by representing a current or magnetic flux of a three-phase induction motor in terms of a vector of a coordinate system known as a d-q coordinate system. The d-q coordinate system is a rotating coordinate system with two orthogonal axes that rotate in synchronization with a power source, one of the axes being taken in a direction of a secondary magnetic flux.
In the vector control, a torque current command value IQR, a magnetic flux current command value IDR, and a slip angular frequency command value xcfx89s* are computed according to the following expressions (1) through (3) using a torque command value T*, a secondary magnetic flux command value "PHgr"2* and a motor constant. A method for deriving the expressions is well known, and described in, for example, xe2x80x9cVector Control of AC Motorxe2x80x9d by Takayoshi Nakano, published by Nikkan Kogyo Shimbunsha; therefore, the description of the method will be omitted herein.                     IQR        =                                            T              *                                      Φ2              *                                xc3x97                      1            P                    xc3x97                      L2            M                                              (        1        )                                IDR        =                              Φ2            *                    M                                    (        2        )                                          ω          ⁢                      xe2x80x83                    ⁢                      s            *                          =                              IQR            IDR                    xc3x97                      R2            L2                                              (        3        )            
where
P: Number of pairs of poles of motor
M: Mutual inductance of motor (H)
L1: Primary self inductance of motor (H)
L2: Secondary self inductance of motor (H)
R2: Secondary resistance value of motor (xcexa9)
Thus, in the vector control, the slip angular frequency command value xcfx89s* is computed according to expression (3) to conduct the control. Expression (3) includes the secondary resistance value R2 of the motor. The value of R2 varies with changes in an ambient temperature or temperature changes caused by heat generated by the induction motor itself. Therefore, for the value of R2 employed for the computation in accordance with expression (3), a value corrected by taking into account a predicted change of R2 caused by a temperature change must be used.
As a known vector controller that takes such a secondary resistance correction into account, there is one disclosed in, for example, Japanese Patent Laid-Open No. 6-343282. FIG. 12 is a block diagram showing a configuration of the known vector controller. The vector controller shown in FIG. 12 includes a vector control unit 49 for controlling an induction motor 55 to be controlled according to a secondary magnetic flux command "PHgr"2* and a torque command TM*, a waveform analyzing unit 50 that receives an induction motor revolution angular velocity xcfx89r (hereinafter referred to simply as xe2x80x9cangular velocity xcfx89r) and performs a waveform analysis on the angular velocity or, a parameter adjusting unit 51 for adjusting a parameter (a set value of the secondary resistance in this example) according to an output of the waveform analyzing unit 50, a subtracter 52 that subtracts the angular velocity xcfx89r from a velocity command xcfx89r* to compute a velocity deviation, and a velocity controller 53 that outputs the torque command value TM* based on a difference between the velocity command xcfx89r* and the angular velocity xcfx89r determined by the subtracter 52 so that the angular velocity xcfx89r follows the velocity command xcfx89r*. The vector controller further includes a power converting unit 54 that controls a primary current value I1 according to a primary current command value I1* output from the vector control unit 49, the induction motor 55 to be controlled that rotates at a predetermined velocity and a predetermined torque according to the primary current value I1, a velocity detector 56 that detects the angular velocity xcfx89r of the induction motor 55, and coefficient setters 57 and 58 for a secondary resistance R2 installed in the vector control unit 49. The following will describe an operation of the related art based mainly on a secondary resistance correction method.
In the related art, a signal that has been step-changed to the velocity command xcfx89r* is input to perform computation for correcting the secondary resistance. A waveform of the angular velocity or when the velocity command xcfx89r* has been step-changed is saved in the waveform analyzing unit 50, and a feature quantity of a response waveform is calculated. The feature quantity calculated by the waveform analyzing unit 50 is supplied to the parameter adjusting unit 51 to calculate a correction amount of the secondary resistance set value R2 so as to correct a set value of the secondary resistance R2 set at the coefficient setters 57 and 58.
FIG. 13 illustrates a configuration example of the waveform analyzing unit 50. A waveform of the angular velocity xcfx89r is sampled by a sample holding circuit 501 and saved in a memory 502, then a feature quantity is calculated by a microprocessor 503. An example of a specific characteristic value employed as the feature quantity and a method for determining the specific characteristic will be discussed later in detail.
A configuration example of the parameter adjusting unit 51 is constituted by a microprocessor and a memory similarly as in the case of the waveform analyzing unit 50 shown in FIG. 13. The figure will be omitted because it is identical to FIG. 13 except for the absence of the sample holding circuit. In this case, an adjustment rule based on a feature quantity is stored in the memory.
Regarding the adjustment of the secondary resistance, the adjustment rule decides a feature quantity to be calculated by the waveform analyzing unit 50 and how a correction amount of the secondary resistance is determined by the parameter adjusting unit 51 by employing the feature quantity. An example of the adjustment rule will be described in conjunction with FIG. 14. FIG. 14 shows simulation results illustrating influences exerted by an erroneous setting of the secondary resistance R2 on a velocity step response waveform. The response waveform is also subjected to influences of a transfer function of the velocity controller 53. Hence, for the purpose of simplicity, in the response waveform of FIG. 14, the transfer function of the velocity controller 53 includes only a proportion factor.
When a true value of the secondary resistance R2 is denoted as R2*, FIG. 14A illustrates a case wherein the value of the secondary resistance R2 set in the vector controller is equal to the true value R2* (R2=R2*). In this case, the torque command TM*, which is an output of the velocity controller 53, and an actually generated torque TM of the induction motor 55 are equal. Therefore, a transfer function of the velocity xcfx89r of the induction motor 55 with respect to the torque command TM* will be determined by the following expression (4), where J denotes a moment of inertia of the induction motor 55, and S denotes a Laplacean.
(xcfx89r/TM*)=(1/Jxc2x7S)xe2x80x83xe2x80x83(4)
Therefore, when a proportion gain of the velocity controller 53 is denoted as GP, a closed loop transfer function Gxcfx89 of the velocity xcfx89r in relation to the velocity command xcfx89r* will be determined by the following expression:
Gxcfx89=(xcfx89r/xcfx89r*)=[GP/Jxc2x7S]/[1+GP/Jxc2x7S]=1/[1+(J/GP)S]xe2x80x83xe2x80x83(5)
The above expression (5) represents a transfer function of a primary delay factor having a time constant expressed as J/GP, and a step response waveform thereof will be represented by the following expression (6), where a step amount of the velocity command is denoted by xcex94xcfx89.
xcfx89r(t)={1xe2x88x92exp[xe2x88x92(GP/J)t]}xcex94xcfx89xe2x80x83xe2x80x83(6)
FIG. 14A illustrates a waveform based on the above expression, FIG. 14B illustrates a case wherein R2 greater than R2* and a value of the secondary resistance R2 set at the vector controller is larger than the true value R2*. In this case, an excessive torque is generated with a consequent overshoot in a response. As a result, a rise is faster than that in a case wherein R2=R2*.
FIG. 14C illustrates a case wherein R2 less than R2*, that is, a value of the secondary resistance R2 set at the vector controller is smaller than the true value R2*. In this case, an insufficient torque is generated, taking a longer time for velocity to reach a final value. As a result, the rise is delayed compared with the case wherein R2=R2*.
Thus, an erroneous setting of the secondary resistance R2 can be visually recognized in the form of a difference in velocity step response waveform. The difference is calculated as a difference in a feature quantity of a response waveform and used for adjusting the secondary resistance.
Diverse quantities can be used as the feature quantities. An example is a time T95 required for the velocity xcfx89r to reach 95% of a step amount xcex94xcfx89, meaning that a rise time is used as a feature quantity. When the setting of the secondary resistance is correct, the response waveform is represented by expression (6), so that a rise time T95* will be a function of a time constant (J/GP) as shown by the following expression:
T95*=(J/GP)ln(20)=2.996(J/GP)xe2x80x83xe2x80x83(7)
As is obvious from FIG. 14, T95 of the step response waveform obtained when R2 greater than R2* is smaller than T95* of the above expression (7), while T95 obtained when R2 less than R2* is larger than T95* of the above expression.
Accordingly, the secondary resistance R2 can be corrected by an adjustment rule described from (1) through (3) below.
(1) The velocity command xcfx89r* is step-changed by velocity control, and the rise time T95 of the velocity xcfx89r during the step change is measured.
(2) If the rise time T95 of the velocity xcfx89r is T95 less than T95*, then R2 greater than R2*; therefore, the value of the secondary resistance R2 set at the vector controller is reduced.
(3) If the rise time T95 of the velocity xcfx89r is T95 greater than T95*, then R2 less than R2*; therefore, the value of the secondary resistance R2 set at the vector controller is increased.
The following will describe a specific example. Referring to FIG. 13, the velocity command xcfx89r* is step-changed by velocity control, and values of response waveforms of the velocity xcfx89r sampled at appropriate sampling cycles during the step change are stored in the memory 502. Based on the response waveforms stored by the microprocessor 503, the rise time T95 is calculated as a feature quantity and supplies the calculation result to the parameter adjusting unit 51. The parameter adjusting unit 51 compares T95 and T95*, and determines a correction amount xcex94R2 of the secondary resistance set value R2 according to expression (8) shown below:
xcex94R2=Kr(T95xe2x88x92T95*)xe2x80x83xe2x80x83(8)
Kr denotes a gain for determining the correction amount from the feature quantity. The value of xcex94R2 thus determined is added to the current secondary resistance set value to calculate a new R2 so as to correct the value set at the vector controller. Thereafter, a step response of the foregoing angular velocity is performed again.
The known vector controller for an induction motor that is capable of automatically adjusting a set value of the secondary resistance of the induction motor has the aforesaid configuration and performs operation as described above. This system allows the set value of the secondary resistance to be adjusted. However, there has been a problem in that a vector controller that does not have the velocity command xcfx89r* is incapable of correcting the secondary resistance according to the known system. There has been another problem in that, even if the vector controller has the velocity command, a special operation has to be performed for correcting the secondary resistance in an application wherein the velocity command is not step-changed during operation.
Thus, the known vector controller described above has not been entirely satisfactory as a vector controller for an induction motor that is capable of automatically adjusting a set value of a secondary resistance.
Accordingly, the present invention has been made with a view toward solving the problems described above, and it is an object thereof to provide a vector controller for an induction motor that is capable of automatically adjusting a set value of a secondary resistance by a simple method without the need for any special signal for adjusting the secondary resistance regardless of the presence of a velocity command.
With the above objects in view, the vector controller for an induction motor of the present invention comprises a vector control command computing means for computing a d-axis current command value IDR, a q-axis current command value IQR and a slip angular frequency command value xcfx89s* based on a torque command value T*, a secondary magnetic flux command value "PHgr"2* and an induction motor constant of an induction motor to be controlled; a revolution angular frequency computing means for computing a revolution angular frequency xcfx89r of the induction motor; an inverter angular frequency computing means for computing an inverter angular frequency xcfx89inv by adding the slip angular frequency command value xcfx89s* and the revolution angular frequency xcfx89r; a dq-axis feed-forward voltage computing means for computing a d-axis feed-forward voltage command value E1DFF and a q-axis feed-forward voltage command value E1QFF by using the d-axis current command value IDR, the q-axis current command value IQR, the induction motor constant and the inverter angular frequency xcfx89inv; a feed-forward voltage vector computing means for computing a feed-forward voltage command value E1F by computing a square root value of a square sum of the d-axis feed-forward voltage command value E1DFF and the q-axis feed-forward voltage command value E1QFF; a feedback current computing means for computing a d-axis feedback current IDF and a q-axis feedback current IQF from a three-phase current value of the induction motor; a compensation voltage computing means for computing a d-axis compensation voltage E1DFB from a difference between the d-axis current command value IDR and the d-axis feedback current IDF, and for computing a q-axis compensation voltage E1QFB from a difference between the q-axis current command value IQR and the q-axis feedback current IQF; a dq-axis voltage command value computing means for computing a d-axis voltage command value E1DR by adding the d-axis compensation voltage E1DFB and the d-axis feed-forward voltage command value E1DFF, and for computing a q-axis voltage command value E1QR by adding the q-axis compensation voltage E1QFB and the q-axis feed-forward voltage command value E1QFF; a voltage vector computing means for determining a voltage command value E1R by computing a square root value of a square sum of each of the d-axis voltage command value E1DR and the q-axis voltage command value E1QR; and a secondary resistance correction value computing means for adjusting a correction value xcex94R2 of a secondary resistance so that a difference between the feed-forward voltage command value E1F and the voltage command value E1R becomes zero. In the vector controller for induction motor of the present invention, the vector control command computing means employs an induction motor constant that has been corrected by the correction value xcex94R2 of the secondary resistance to compute the slip angular frequency command value xcfx89s* to thereby conduct vector control of the induction motor.
Alternatively, the vector controller of the persent invention may be provided with a slip angular frequency correction value computing unit 30 in place of the secondary resistance correction value computing unit.